The objective of this proposal is to study the enzyme purine nucleoside phosphorylase from lymphoid cells and other human cells in an attempt to establish the relationship between enzymatic deficiency and immunodeficiency. The presence and the levels of the enzyme in separated T and B lymphocytes and monocytes from human peripheral blood will be determined as well as enzyme changes that may accompany stimulation of the above cells by mitogens andantigens. The enzyme will be purified from separated T and B lymphocytes, monocytes, blasts and macrophages and will be analyzed by two dimensional gel electrophoresis to determine the state of subunit modification. The effect of subunit modification on the kinetic and catalytic properties of the enzyme will be assesed by isoelectric separation of the subunits of the erythrocytic enzyme (in the presence of 4 M urea) and reconstruction of homotrimers composed of only one type of subunit.